In multicolor printing machines, an accurate overprint of the individual subimages must be ensured, each subimage being formed of one color.
Printing machines are equipped with means permitting a shifting of the individual images in such a way that they occupy the respectively desired position. An automatic control of this operation is achieved with register control means. However, the image is translationally moved in its position relative to the other subimages in circumferential and lateral direction, which creates a positioning error.
In roll type printing series machines, a further flaw is observed that is superposed on the above-mentioned positioning error. This flaw is due to the fact that a material web is not conveyed in an exactly straight way through a printing unit for different reasons. For instance, when a printing material web is thinner at one side than at the other side, the web is pulled through the printing unit at one side only. This has the effect that, although the printing operation is carried out in an accurate position for selected positions, it is not performed in edge parallelism with the preceding print.
It is therefore standard practice, for instance in an intaglio printing machine, to guide the web in front of each printing unit over a feed roller which can be changed in its axial position relative to the axis of the plate cylinder or the imaginary printing line. With a suitably inclined position of the roller axis, this has the effect that an edge-parallel print with respect to the pre-print or an edge-parallel entry into the printing unit is now achieved.
In the formerly known methods for adjusting the feed roller, each printing unit is equipped with a manually adjustable feed roller for compensating for tolerances that might be due to the material to be printed or due to the setting of the printing machine.
This roller is imperative for the reasons explained above and is positioned in front of the printing cylinder in the printing unit. The parallelness of the pre-print is adjusted by inclination of the roller.
The parallelness of the print is basically adjusted or set during production by the printing machine. The visual control is normally carried out through a web monitoring system, which is usually installed after the last printing unit.
It is determined through this visual control whether in various printing units the feed roller has to be adjusted. In practice, the feed roller must be corrected in almost every printing unit to achieve perfect parallelness.
However, an exact manual adjustment in accordance with the flaw size is very difficult to perform and is normally not immediately successful. Hence, the feed roller must be corrected several times before achieving adequate parallelness. This adjusting process is very time-consuming and is prolonged by the fact that the printing machine must travel long distances between the place of displacement and the position for monitoring the result. Hence, waste is produced during this relatively long period of time. Since today's modern printing machines have eight to ten or even more printing units as a rule, and since a correction of the feed rollers may be required within each roll or after the rolls have been changed, the non-detection of inadequate parallelness is tantamount to a significant lack of quality which leads to a considerable amount of waste, and a considerable amount of time is additionally needed for a manual correction, which produces additional waste.
What is needed, therefore, is a method and an apparatus for measuring, setting or controlling longitudinal and lateral register as well as parallelness of the printing register in a multicolor printing machine, whereby the necessary setting work can be simplified and shortened and the amount of waste can thus be minimized.